1. Field of the Invention
The present invention relates to a direct light type backlight unit and a large-sized liquid crystal display that does not include a color filter.
2. Description of the Related Art
In general, liquid crystal displays (LCDs), which are flat panel displays used in lap top computers, desktop computers, LCD TVs, and mobile communication terminals, are light receiving devices which do not form images by themselves, but by receiving light from an external source, and need a backlight unit in addition to a liquid crystal panel. A backlight unit is installed behind a liquid crystal display and serves as a light source to emit light.
According to the arrangement of a light source, the backlight unit can be classified as either a direct light type backlight unit in which a plurality of lamps installed directly under a liquid crystal display emit light directly to a liquid crystal panel, or an edge light type backlight unit in which a lamp installed at an edge of a light guide panel (LGP) emits light and the light is transferred to a liquid crystal panel.
When a color filter is used to realize a color image, light efficiency is decreased and manufacturing costs of a liquid crystal display are increased. In order to solve this problem, a color image forming technique of applying a grating instead of using a color filter to separate colors has been suggested.
FIG. 1 illustrates a conventional liquid crystal display using an edge light type backlight unit. The liquid crystal display includes a light source 10 and a light guide panel 15 guiding the light from the light source 10 onto the liquid crystal panel 30. A cold cathode fluorescent lamp (CCFL) is used as a light source, but a CCFL has a low color realization rate and thus does not provide high image quality and is not appropriate for high resolution TVs or monitors. The light guide panel 15 is tapered to guide the light emitted from the light source 10 efficiently and distribute the light uniformly.
A reflection panel 20 is formed below the light guide panel 15 to reflect the light transmitted from the light guide panel 15 back to the light guide panel 15. A grating 23 is formed on the upper surface of the light guide panel 15 to diffract incident white light at different angles according to wavelength to separate the white light. It is important for the light to be incident at an optimal angle of incidence for maximum diffraction efficiency to improve the light efficiency. However, it is difficult to minutely control the angle of incidence of the light which is guided by the light guide panel 15 and incident on the grating 23.
The color light emitted from the grating 23 is focused on the liquid crystal panel 30 through a cylindrical micro lens array 25. The grating 23 diffracts light, for example, red light (R), green light (G), and blue light (B), at different diffraction angles so that the light is incident on corresponding pixels of the liquid crystal panel 30. The liquid crystal panel controls the colored light by switching the pixels on and off according to input image signals to form a color image. A diffuser 35 diffuses the image formed by the liquid crystal panel 30 to increase a viewing angle.
As described above, in the edge light type backlight unit, it is difficult to control light to be incident on a grating at an optimal angle of incidence. Since light is deflected from one direction to another by a light guide panel, light uniformity may not be secured. Thus, the size of liquid crystal displays is limited.